Method of calendering a paper web or equivalent

ABSTRACT

A method for calendering a web such as a paper web in a multi-nip calender or supercalender, in which the web is passed through calendering nips formed by rolls placed in two or more stacks of rolls. The web is passed alternatingly from the corresponding calendering nip in each stack of rolls into the calendering nip in the following stack of rolls. In other words, the web is passed from a first calendering nip in a first stack of rolls to a first calendering nip in a second stacks of rolls, after the first calendering nip in the second stack of rolls to a second calendering nip in the second stack of rolls, from the second calendering nip in the second stack of rolls to a second calendering nip in the first stack of rolls, and after the second calendering nip in the first stack of rolls to further processing.

FIELD OF THE INVENTION

The invention concerns a method in calendering, in particularsupercalendering, of a paper web or equivalent, in which method thepaper web or equivalent is passed through the calendering nips formed byrolls for calendering of the paper web, in which method the rolls areplaced in at least two stacks of rolls, in which method the paper web ispassed from a corresponding calendering nip in each stack of rolls intoa corresponding calendering nip in the next stack of rolls.

BACKGROUND OF THE INVENTION

As is well known, the set of rolls in a supercalender comprises a numberof rolls, which have been arranged one above the other as a stack ofrolls. The rolls placed one above the other are in nip contact with eachother, and the paper web to be calendered is arranged to pass throughthe nips between the rolls. Supercalenders involve the drawback that insupercalenders the nips are loaded by the force of the weight of the setof rolls, in which case the distribution of the linear load from theupper nip to the lower nip is increasing in a substantially linear way.This has the consequence that the linear load present in the lower nipdetermines the loading capacity of the calender. Thus, the calender isdimensioned in compliance with the performance of the lowest rolls and,at the same time, some of the loading or calendering potential of theupper nips remains unused. Earlier, attempts have been made to solvethis drawback related to the prior art so that attempts have been madeto increase the deficient loading of the upper nips so that thesupercalender is placed in the horizontal plane or that the stack ofrolls in the calender is divided into two stacks of rolls. Such anembodiment has, however, also proved expensive, because a calender oftwo parts requires a higher number of adjustable-crown rolls. Oneembodiment in which the set of rolls in the calender has been dividedinto two stacks of rolls is described in the DE Utility ModelApplication No. 295 04 034.3. In this prior-art solution, in thecalender divided in two stacks of rolls, the paper web is first passedthrough the calendering nips in one stack of rolls, and after that thepaper web is passed into the other stack of rolls, in which the paperweb is passed through its calendering nips.

In the way known from the prior art, in supercalenders, first one side,for example the top side, of the paper web has been calendered/glazed,and then the other side, for example the bottom side. In the prior-artsolutions, a so-called reversing nip has determined where the side to beglazed is changed. In supercalenders, the glazing takes place mostintensively in the first nips, in which case the glazing of the sidethat is glazed in the later nips to the same glaze level as is reachedby the side glazed first requires, relatively speaking, higher linearloads, and this causes a higher loss of bulk in the web. Also, thedifficult control of successive glazing often produces unequalsidednessin the paper glaze degree.

OBJECTS AND SUMMARY OF THE INVENTION

The object of the present invention is to provide a solution in whichthe problems related to unequalsidedness of glazing have been eliminatedor at least minimnized.

In view of achieving the objectives stated above and those that willcome out later, the method in accordance with the invention is mainlycharacterized in that, in the method, the paper web is passed from onestack of rolls into the next stack through a corresponding calenderingnip in each stack of rolls until the web is passed from the last stackof rolls back through the following corresponding calendering nips fromone stack of rolls into the next stack in the reversed sequence, andthis is repeated until the paper web is passed from the last calenderingnip to further processing.

According to the invention, the set of rolls in the supercalender hasbeen divided into at least two stacks of rolls placed at a distance fromone another, wherein the web runs from one stack of rolls into the nextone until it is returned from the last stack of rolls through the otherstacks of rolls into the first stack, etc. Thus, in the stacks of rollsin the calender, the web runs first through, for example, the firstcalendering nips in all the stacks of rolls and next through the secondcalendering nips in the reversed sequence until the web is passed fromthe last calendering nip to further processing, for example to reeling.This provides a number of advantages. In particular, the problemsrelated to unequalsidedness of glazing are reduced remarkably and,moreover, the control of unequalsidedness is easier. Further, thesolution in accordance with the invention provides savings in the bulkof the web, because the intensively loaded lower nips at the side thatis to be glazed last are omitted and the glazing of both sides isstarted at the same time. By means of the present invention, a betterglaze and smoothness are also obtained for the paper, and, moreover, itis possible to use lower linear loads, in which case the rolls have alonger service life and it is also possible to use higher runningspeeds. The arrangement in accordance with the invention does, initself, not require a higher number of rolls than in the prior-artsolutions, because by means of the mode of running in accordance withthe invention and by means of the arrangement of regulation of thetension the same quality level is achieved with a lower number of nipsthan in the prior-art solutions.

According to the invention, the paper web can also be passed, in steadof being passed into the topmost calendering nip in a stack of rolls,for example, first into the lowest nip in the stack of rolls or eveninto the second nip from the top or from the bottom. The latteralternative is suitable for use, for example, when it is desirable, forexample when the first treatment nip is at the top, to bring the bottomside of the web first into contact with a hot roll face and when, at thesame time, it is desirable to use non-coated upper rolls only. In such acase, in the nip between the upper roll and the soft roll placed belowit, no web runs at all. A similar situation can, of course, also arisewhen soft-coated upper/lower rolls only are used.

Also, a supercalender construction consisting of at least two stacks ofrolls is of lower height, in which case an equally large space in thevertical direction is not needed as in the prior-art supercalendersolutions. Nor is a reversing nip needed in the invention in order toreverse the other side of the web for glazing, but the rolls in thestacks of rolls have been arranged so that both sides of the web areglazed substantially at the same time.

According to the invention, for the control of the web tension betweenthe stacks of rolls, for example, linear loads adjustable specificallyfor each nip are employed. The tensions and the differences in speed canalso be controlled by choosing different roll hardnesses or rollmaterials or by using roll-specific drives.

For the control of the web tensions between stacks of rolls, it is alsopossible to utilize the stretch, in which the lag arising from slippingproduces a slower surface speed of the upper rolls. The stretch arisingfrom compression of paper compensates for the lag arising from slipping,and with a certain paper with a set of rolls of a certain sort the lagand the stretch are highly stable, i.e. the same paper can also be runthrough two separate equal sets of rolls.

For the control of web tensions, it is also possible to use nip-specificregulation of the linear load. For example, if a slack portion or anexcessive tension tends to be formed in some gap between nips, thedifference in draw over said gap can be changed by tightening orslackening the preceding or following nip. In the arrangement inaccordance with the invention, it is also relatively easy to find astate of balance in which the tensions over all gaps are substantiallyequal, and in such a case the same glaze is obtained for both sides ofthe paper. The nip load, the roll hardness, and roll-specific drives,all of them affect the difference in speed between the rolls, in whichconnection all of these factors can be utilized for the control of thetensions between the stacks of rolls.

If necessary, the web tension can be measured between the stacks ofrolls, and based on the measurement results it is possible to regulatethe difference in speed.

Further, in the present invention, it is possible to take advantage ofthe fact that paper stretches when it becomes moist and shrinks when itbecomes dry. In the present invention, this phenomenon is utilized inthe control of the tension of the paper between the nips by regulatingthe moisture balance to such a level that the run of the web between thenips is controlled.

In a summarizing way, it can be stated that, in an arrangement inaccordance with the present invention, the runnability is achieved sothat a predetermined web tension is maintained by regulating the nippressures specifically for each nip, for example, by means of reliefdevices or by in the sets of rolls placing self-loading rolls also inintermediate positions, by regulating the torques applied to the rollsthrough their drives or by regulating the surface speeds of the rolls,by regulating the temperature and/or moisture of the web, by regulatingthe surface temperature and/or the inside temperature in the rolls, andby regulating the temperature, pressure and humidity in the atmospheresurrounding the web. The predetermined web tension is maintained bychoosing the properties of the roll coatings so that slipping betweenthe rolls maintains the tensions of the web portions placed between thestacks of rolls.

In the arrangement in accordance with the invention, the calenderingresult is regulated first by regulating the properties of the web:glaze, thickness and smoothness, which are regulated by using theregulation parameters mentioned above in relation to the regulation oftension. Thus, in the regulation of the stacks of calender rolls,consideration is given, at the same time, both to control parametersrelated to the regulation of the web tension and to control parametersrelated to the web properties aimed at.

In the regulation of the regulation parameters, it is possible to usethe following methods and devices. The web tension is regulated by meansof empirical running parameters that have been stored in the memoryspecifically for each paper grade, or adaptively by means of devices ofmeasurement of tension, of which devices can be mentioned tensionmeasurement rolls, sets of rolls for tension measurement, measurement oftension by means of an air-cushion beam, measurement of tension as pointmeasurement by means of an oscillating or static detector. Regulation ofthe roll and web speeds can be carried out based on measurement ofsurface speed, on measurement of speed of rotation of the rolls, orbased on measurement of the flow in the roll drives. In the regulationof the roll temperatures, it is possible to use internal heat regulationmethods, for example regulation of the heat transfer by a heatingmedium, for example regulation of the heat transfer by means of oil,water, steam, combustion gases, heating by means of an electricresistor, or inductive heating. In the regulation of the rolltemperature, it is also possible to use external heat regulationmethods, such as inductive heating, radiation heating and/or gasblowing.

In regulation of the temperature/heating capacity of the rolls, adaptiveregulation connected with measurement of the properties of the web,adaptive regulation connected with measurement of the roll surfacetemperature, adaptive regulation connected with measurement of thetemperature of the roll heat regulation medium, or adaptive regulationconnected with measurement of the heating capacity of a roll is used.

The moisture and the temperature of the web can be regulated byadjusting the feed of steam onto the web, by regulating the feed ofwater mist onto the web, by means of drying/heating of the web producedby means of air blowing, by means of moistening taking place by means ofa film transfer method, or by means of regulation of the atmospheresurrounding the web. Besides by the means mentioned above, the webtemperature can also be regulated by means of heat regulation rolls andsets of belts, by means of sets of belts, by means of radiation heaters,and by means of regulation of the deflection angle of the web againstthe calender rolls.

According to the invention, the stacks of rolls used in calenders can beprovided with most different combinations of rolls. Possiblecombinations include, for example, stacks of rolls in which all therolls are provided with soft coatings, in which all the rolls areprovided with hard faces, for example metal-faced or ceramic-faced rollsor equivalent, or in which some of the rolls are soft rolls and some ofthem hard rolls. As a special case can be mentioned a stack of rolls inwhich every other roll is soft and every other roll hard.

Further, in the stacks of rolls in a calender, all the rolls can bedriven rolls, or only one roll is driven, as is known from conventionalsupercalenders. When the stacks of rolls are operated by means of onedrive roll only, the location of the drive roll is chosen so that theslipping between said roll and the other stacks of rolls in the set ofrolls maintains the tension of the web portions between the stacks ofrolls and permits regulation of tension by means of other regulationparameters.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following the invention will be described in more detail withreference to the figures in the accompanying drawing, in which

FIG. 1 is a schematic illustration of a prior-art calender whichconsists of two stacks of rolls,

FIG. 2 is a schematic illustration of an exemplifying embodiment of acalender in accordance with the present invention,

FIG. 3 is a schematic illustration of a second exemplifying embodimentof a calender in accordance with the present invention,

FIG. 4 is a schematic illustration of a third exemplifying embodiment ofa calender in accordance with the present invention,

FIGS. 5A, 5B and 5C are schematic illustrations of some embodiments forpassing the paper web through a calender consisting of two stacks ofrolls,

FIG. 6 shows a control block diagram for a calender of the presentinvention.

DETAILED DESCRIPTION OF THE PREFERRED

In the prior-art calender 10' shown in FIG. 1, the calender comprisestwo stacks 10₁ ',10₂ ' of rolls, in which solution the paper web W' isfirst passed through the calendering nips N' in the first stack 10₁ ' ofrolls and after that through the calendering nips N' in the second stack10₂ ' of calendering rolls. The various rolls in the calender aredenoted with the reference numerals 11',12',13',14',15',18',19',20',21',22'. The guide rolls of the web are denoted with the referencenumerals 23', and the frame constructions of the calender are shown inthe figure schematically, and they are denoted with the referencenumeral 30'.

In the embodiment of the invention shown in FIG. 2, the calender 10comprises two stacks 10₁,10₂ of rolls, and calender rolls11,12,13,14,15,16;17,18,19,20,21,22 are fitted one above the other ineach of said stacks. In the roll stack 10₁,10₂, hard rolls11,13,15;18,20,22 and soft rolls 12,14,16;17,19,21 alternate, and theweb W is passed to run alternatingly through the calendering nipsN₁₂,N₁₃,N₁₄,N₁₅ ;N₂₁, N₂₂,N₂₃,N₂₄,N₂₅ in each stack 10₁ ;10₂ of rolls sothat different sides W₁,W₂ of the web W are glazed alternatingly.

As is shown in FIG. 2, the paper web W is passed through the nip N₁₁between the hard roll 11 and the soft roll 12 in the first stack 10₁ ofrolls and through the nip N₂₁ formed by the topmost soft roll 17 and thehard roll 18 in the second stack 10₂ of rolls onto the web guide roll23, by whose means the web is reversed and passed through the secondhighest nip N₂₂ in the second stack 10₂ of rolls formed between the hardroll 18 and the soft roll 19 into the first stack 10₁ of rolls, in whichthe web W is passed through the second highest nip N₁₂ between the softroll 12 and the hard roll 13 onto the web guide roll 24. In this way thepaper web W runs from one stack of rolls into the other through the nipsN₁₃,N₂₃,N₂₄,N₁₄,N₁₅,N₂₅ so that both sides W₁,W₂ of the web W are glazedalternatingly. The frame constructions of the calender 10 areillustrated in the figure just as a schematic construction 30. The drivearrangement of the roll 11 is illustrated as a schematic illustration31. The tension of the paper web W can be measured in the area betweenthe stacks 10₁,10₂ of rolls, for example, by means of a measurementdevice 32,33, from which the measurement result is passed to the controland drive system 34,35 of the calender, which system 34,35 controls theoperation of the calender, on the basis of the measurement result, sothat the web tension is at the desired level.

FIG. 3 is a schematic illustration of an exemplifying embodiment of theinvention in which steam boxes 40 have been fitted in connection witheach stack 10₁,10₂ of rolls for moistening of the paper web W. In theother respects the exemplifying embodiment shown in FIG. 3 is similar tothat shown in FIG. 2.

FIG. 4 shows an exemplifying embodiment of the invention in which thecalender 10 is composed of three stacks 10₁,10₂ and 10₃ of rolls. In theexemplifying embodiment shown in the figure the paper web W is firstpassed through the first calendering nip N₁₁,N₂₁,N₃₁ at the top of eachstack 10₁,10₂,10₃ of rolls, after which the paper web W is passed overthe guide roll 23 through the second calendering nip N₃₂,N₂₂,N₁₂ in eachstack of rolls in the reversed sequence, and so forth, until the paperweb W is passed through the last calendering nip N₃₅ to furtherprocessing. In the other respects the calender 10 is similar to theexemplifying embodiments shown in FIGS. 2 and 3, and corresponding partsare denoted with corresponding reference numerals. The calender rolls inthe stack 10₁ of rolls in the calender 10 are denoted with the referencenumerals 11,12,13,14,15,16, the calender rolls in the second stack 10₂of rolls in the calender 10 are denoted with the reference numerals17,18,19,20,21 and 22, and the calender rolls in the third stack 10₃ ofrolls are denoted with the reference numerals 41,42,43,44,45 and 46. Theframe constructions of the calender are denoted with the referencenumeral 30, and the measurement, regulation and/or drive system of thecalender with the reference numerals 32,34.

FIGS. 5A, 5B and 5C illustrate some applications for running the paperweb W in a calender 10. The running direction of the paper web W isdenoted with the arrow F, and the web W is passed alternatingly throughthe calendering nips in each stack 10₁,10₂ of rolls.

In FIG. 5A the paper web W is passed into the second calendering nip,counted from the top, in the first stack 10₁ of rolls, which is formedbetween the rolls 12 and 13.

In FIG. 5B the paper web W is passed into the calender 10 first into thesecond calendering nip, counted from the bottom of the stack, in thefirst stack 10₁ of rolls, which nip is formed between the rolls 14 and15.

In the exemplifying embodiment shown in FIG. 5C the paper web W is firstpassed into the lowest calendering nip in the first stack 10₁ of rolls,which nip is formed between the rolls 15 and 16.

In the control diagram shown in FIG. 6, the control system consists ofthe following parts: paper tension measurement 51, paper moisturemeasurement 52, paper speed measurement 53, paper temperaturemeasurement 54, paper thickness measurement 55, paper glaze measurement56, roll speed measurement 57, roll surface temperature measurement 58,roll internal temperature measurement 59, measurement of heatingcapacity of rolls 60, measurement of drive capacity of rolls 61,measurement of drive torque of rolls 62, on the basis of whichmeasurements the regulation values are determined by means of thepaper-grade specific running parameters 63, and the regulation valuesare determined 64 by means of the mapped process, on whose basis theregulation 65 of the actuators that act upon the moisture of the paper,the regulation 67 of the drives, the regulation 67 of the properties ofthe atmosphere surrounding the paper, and the regulation 68 of thetemperature of the rolls are obtained. For the control system, there canbe a number of measurement points that is chosen freely based on therequirements and placed at suitable locations.

Above, the invention has been described with reference to some preferredexemplifying embodiments of same only, and the invention is by no meanssupposed to be strictly confined to the details of said embodiments.Many variations and modifications are possible within the scope of theinventive idea defined in the following patent claims.

We claim:
 1. A method for calendering a web, comprising the stepsof:arranging rolls in at least two stacks and to define calendering nipsbetween adjacent ones of the rolls in each of the at least two stacks ofrolls, passing the web from a first one of the calendering nips in afirst one of the at least two stacks of rolls to a first one of thecalendering nips in a second one of the at least two stacks of rolls,passing the web after the first calendering nip in the second stack ofrolls through a second one of the calendering nips in the second stackof rolls, passing the web from the second calendering nip in the secondstack of rolls to a second one of the calendering nips in the firststack of rolls, passing the web after the second calendering nip in thefirst stack of rolls to further processing; measuring tension of the webin runs between the first and second stacks of rolls, and utilizing themeasured tension to regulate the calendering of the web in thecalendering nips.
 2. The method of claim 1, wherein the at least twostacks of rolls consists of only two stacks of rolls.
 3. The method ofclaim 2, wherein the calendering nip in the first and second stacks ofrolls are formed such that in each stack of rolls, a hard calender rolland a soft calender roll alternate whereby one side of the paper isglazed alternatingly.
 4. The method of claim 2, further comprising thesteps of:passing the web from the second calendering nip in the firststack of roll to a third one of the calendering nips in the first stackof rolls, and passing the web from the third calendering nip in thefirst stack of rolls to a third one of the calendering nips in thesecond stack of rolls and then to further processing.
 5. The method ofclaim 1, further comprising the step of:constructing the rolls in thefirst and second stacks of rolls to have a certain hardness and be of acertain material to enable the web to have a desired tension.
 6. Themethod of claim 1, further comprising the step of:regulating tension ofthe web between the stacks of rolls by means of roll-specific drives. 7.The method of claim 1, further comprising the step of:regulating tensionof the web by using the stretch and compression of the web.
 8. Themethod of claim 1, wherein each side of the web is calenderedsubstantially simultaneously.
 9. The method of claim 1, wherein insuccessively arranged ones of the calendering nips, a different side ofthe web is calendered.
 10. The method of claim 1, wherein the at leasttwo stacks of rolls comprises first, second and third stacks of rolls.11. The method of claim 10, further comprising the steps of:passing theweb from the first calendering nip in the second stack of rolls to afirst calendering nip in the third stack of rolls, passing the web fromthe first calendering nip in the third stack of rolls to a second one ofthe calendering nips in the third stack of rolls, and passing the webfrom the second calendering nip in the third stack of rolls to thesecond calendering nip in the second stack of rolls.
 12. The method ofclaim 11, wherein the first calendering nip in the first, second andthird stacks of rolls are positioned such that the web runs in asubstantially horizontal path from the first calendering nip in thefirst stack of rolls to the first calendering nip in the second stack ofrolls and from the first calendering nip in the second stack of rolls tothe first calendering nip in the third stack of rolls.
 13. The method ofclaim 11, wherein the first calendering nip in the first and secondstacks of rolls are positioned such that the web runs in a substantiallyhorizontal path from the first calendering nip in the first stack ofrolls to the first calendering nip in the second stack of rolls.
 14. Themethod of claim 1, wherein the first calendering nip in the first andsecond stacks of rolls is the uppermost calendering nip in therespective stack of rolls.
 15. The method of claim 1, wherein the firstcalendering nip in the first and second stacks of rolls is the lowermostcalendering nip in the respective stack of rolls.
 16. The method ofclaim 1, wherein the first calendering nip in the first and secondstacks of rolls is an intermediate calendering nip in the respectivestack of rolls between the uppermost and lowermost calendering nips. 17.A method for calendering a web, comprising the steps of:arranging rollsin at least two stacks and to define calendering nips between adjacentones of the rolls in each of the at least two stacks of rolls, passingthe web from a first one of the calendering nips in a first one of theat least two stacks of rolls to a first one of the calendering nips in asecond one of the at least two stacks of rolls, passing the web afterthe first calendering nip in the second stack of rolls through a secondone of the calendering nips in the second stack of rolls, passing theweb from the second calendering nip in the second stack of rolls to asecond one of the calendering nips in the first stack of rolls, passingthe web after the second calendering nip in the first stack of rolls tofurther processing, and regulating tension of the web by regulating thelinear load in at least one of the calendering nips in the first andsecond stacks of rolls.
 18. A method for calendering a web, comprisingthe steps of:arranging rolls in at least two stacks and to definecalendering nips between adjacent ones of the rolls in each of the atleast two stacks of rolls, passing the web from a first one of thecalendering nips in a first one of the at least two stacks of rolls to afirst one of the calendering nips in a second one of the at least twostacks of rolls, passing the web after the first calendering nip in thesecond stack of rolls through a second one of the calendering nips inthe second stack of rolls, passing the web from the second calenderingnip in the second stack of rolls to a second one of the calendering nipsin the first stack of rolls, passing the web after the secondcalendering nip in the first stack of rolls to further processing,measuring a point between the first and second stacks of rolls at whichtension of the web is changed, and changing the tension of the web bytightening or slackening the calendering nip preceding or following thepoint at which the change in the tension of the web is measured.
 19. Amethod for calendering a web, comprising the steps of:arranging rolls inat least two stacks and to define calendering nips between adjacent onesof the rolls in each of the at least two stacks of rolls, passing theweb from a first one of the calendering nips in a first one of the atleast two stacks of rolls to a first one of the calendering nips in asecond one of the at least two stacks of rolls, passing the web afterthe first calendering nip in the second stack of rolls through a secondone of the calendering nips in the second stack of rolls, passing theweb from the second calendering nip in the second stack of rolls to asecond one of the calendering nips in the first stack of rolls, passingthe web after the second calendering nip in the first stack of rolls tofurther processing, and regulating tension of the web by regulating themoisture balance of the web.